The present invention relates to a system for transmitting emergency calls from persons requiring assistance.
Such a system is disclosed in European patent application No. 21,425, filed June 25, 1980, wherein an emergency calling station connected to a subscriber line of a dial telephone exchange system automatically calls the number of an emergency center, likewise connected to the dial telephone exchange system, upon actuation of a sensor by the person requiring assistance. Bidirectional data transmission, by way of frequency keying, takes place in the voice band over the thus established connection. A two-way voice communication device in the emergency calling station can be switched into operation by an operating mode control signal from the emergency center. The direction of voice transmission is then switched by a voice transmission direction control signal.
The above described system has the drawback that data and voice transmission interfere with one another because the signals of both transmission modes are transmitted, via a summing amplifier and a branching circuit, over the same telephone connection.
The simultaneous transmission of voice and data over a single channel without mutual interference is known as the so-called carrier telegraphy and is described, for example, in the book "Hutte IV B, Fernmeldetechnik" [Communication Art], 28th Edition, Berlin and Munich, 1962, page 730. Its drawback is that because of the subdivision of the entire band into a voice channel and a telegraph channel by means of frequency filters, the clarity of the voice transmission and the data transmission rate are less than if each had available the full bandwidth.
The alternating transmission of voice and data over the same telephone connection by switching off the speaking device with an operating mode control signal transmitted over the telephone connection and switching on the data device is known, generally, from German Pat. No. 910,794. This known arrangement, however, has the disadvantage that the switching between data transmission and voice transmission is relatively slow.